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Scientists in the US are planning to turn a cubic kilometre of the Antarctic ice sheet into a giant ‘neutrino telescope’. Neutrinos are elusive particles which interact with matter so rarely that large bodies of material are often needed to detect them. The neutrino telescope will detect light emitted by high-energy neutrinos as they interact with the ice. ‘It’s a fantastic way to do neutrino astronomy, much cheaper than anything anyone ever dreamed of,’ says Francis Halzen of the University of Wisconsin. ‘We are looking for things like active galactic nuclei which put out tremendous energies in neutrinos.’ Halzen’s detector will be a collaboration between the University of Wisconsin and the University of California at Irvine and Berkeley. It will use a similar technique to that currently being used by a Japanese group, which is monitoring solar neutrino reactions with water. Halzen’s proposed detector will observe neutrinos from cosmic sources with energies more than 1000 times greater. The neutrino telescope will use phototubes to watch for light produced by neutrino reactions. The Japanese suspend their phototubes in the water. Halzen plans to put his phototubes into holes melted in the ice sheet, then let them freeze in place. He says the ice will cool the phototubes, making them more sensitive, and also provide an environment free of natural radioactivity. Halzen has carried out preliminary tests on the Greenland ice sheet. The National Science Foundation has provided him with funds to drill 10 test holes near the American station at the South Pole. One of his goals will be to map the optical quality of the ice so that he can be sure that light from neutrino reactions will indeed reach the detectors. This will involve measuring how deep below the surface ice bubbles go. If the Wisconsin group can demonstrate that their scheme works, they could go back every year to make more holes and enlarge the detector. An array a square kilometre in size should see neutrino sources if there are any,